Implantable cardiac devices include implantable defibrillators, implantable pacemakers, and implantable monitoring devices (and combinations thereof). Such devices monitor the cardiac status of a patient by observation of signals captured with implanted electrodes. Interference with cardiac signal sensing can come from many sources including biological noise (muscle contraction and artifacts of respiration or other movement, for example) and external noise (50/60 Hz line frequency noise, electrical devices, radiofrequency generators, or large magnets, for example). Noise introduced by any of these sources can prevent detection of treatable conditions and/or cause the device to deliver inappropriate therapy. New and alternative ways of detecting and responding to such noise are desired.